Meteorological Factors Affecting NO₂ Pollution In Nanjing And Numerical Simulation Of Typical Case

In recent years,with urbanization and economic development,the number of motor vehicles has increased rapidly,the total nitrogen oxide emission has increased,and the problem of NO₂ pollution has become increasingly serious,posing a serious threat to human health and the ecological environment.In this paper,Nanjing was taken as the study area,and the annual,monthly,seasonal and diurnal variations of NO₂ concentration in Nanjing were statistically analyzed by using hourly observation data of NO₂ concentration from environmental monitoring stations and daily meteorological data of Nanjing station,and the generalized additive model（Generalized Additive Models,GAM）between NO₂ concentration and meteorological factors was constructed.The influence characteristics of meteorological factors on the change of NO₂ concentration were quantitatively studied.On this basis,WRF-Chem model was used to conduct numerical simulation of a single case of NO₂ pollution and analyze the meteorological causes of the pollution process.HYSPLT-4 model was used to simulate the transport path of NO₂ pollution and explore the distribution characteristics of potential sources of NO₂ pollution.The following results are obtained:（1）From 2017 to 2019,the overall NO₂ concentration in Nanjing showed a decreasing trend year by year,but by 2019,there were still more than 15 days of NO₂ concentration exceeding standard monitored at several stations.The monthly variation of NO₂ concentration ranged from 24.04μg/m³ to 52.66μg/m³,with the lowest value appearing in August and the highest value in December.In terms of seasons,NO₂ concentration reached its lowest value in summer and its highest value in winter,and the concentration in spring and autumn was between them.（2）The concentration of NO₂ presented a"double peak phenomenon"on weekdays,and a"single peak phenomenon"on holidays.The concentration of NO₂ on weekdays was higher than that on holidays.There was a significant difference between the two concentrations at7:00～14:00 and 19:00～23:00,and the difference reached the maximum at 9:00 am,which was14.34μg/m³.（3）GAM model can well explain the response of NO₂ concentration to meteorological factors.NO₂ concentration increased first and then decreased with the increase of average temperature,relative humidity and average air pressure,and decreased with the increase of average wind speed in a logarithmic manner.Under the synergistic effect of average temperature around 10℃+relative humidity around 52%+low wind speed+average pressure around 1010 h Pa,NO₂ accumulation was the most favorable.（4）In the case of NO₂ pollution,the emergence of multiple inversion layers,the downward movement of the low value area of horizontal wind speed and the weak vertical velocity made NO₂ accumulate continuously.The external sources of NO₂ pollution were mainly transported by nearby short-distance air flow and internal air flow in Jiangsu province.The central part of Hefei,the east of Lu’an city,most areas of Zhenjiang City,the northeast of Changzhou city and the junction area between Nanjing and Zhenjiang city were the main source areas,and the WPSCF values of both source areas were greater than 0.9.WCWT values ranged from 90μg/m³ to 100μg/m³.